1. Field of the Invention
The present invention relates to a display device and, more particularly, to a horizontal retrace time adjustment pulse generating circuit for a display device, in which the retrace time of the horizontal deflection circuit is properly adjusted with respect to the flyback pulse.
2. Discussion of Related Art
An exemplary display device comprises a video preamplifier which amplifies a video input by means of a low-voltage amplifier and maintains the amplified signal at a predetermined level; a video main amplifier which amplifies the output from the video preamplifier and supplies the amplified signal energy for each pixel of a cathode-ray tube (CRT); a microcomputer which receives horizontal and vertical synchronizing signals to produce screen control signals; vertical and horizontal deflection circuits which receive the screen control signals and generate vertical and horizontal deflection signals for output to a deflection yoke mounted on the CRT; and a high voltage circuit which supplies the anode voltage for the CRT, using a flyback pulse generated by the horizontal deflection circuit.
The vertical and horizontal deflection signals control the deflection of an electron beam generated from an electron gun within the CRT, to thus form a displayed image on the screen or the display device. The horizontal scanning in which the electron beam is deflected horizontally and vertical scanning in which the electron beam is deflected vertically are performed concurrently.
In horizontal scanning, after completing the scanning of a horizontal line in the display area, the next line is scanned from a point just below that previously scanned. The time duration from the completion of the previous line to the beginning of the next is commonly called retrace time, or horizontal flyback time. To display a clear image void of retrace lines, the video signal is blanked during the retrace time. Thus, the display device is provided with a retrace time signal blanking circuit for generating a blanking signal to remove the retrace line caused by the horizontal flyback of the electron beam for scanning the next line. If the retrace line signal is not properly blanked, video noise appears on the display.
General design specifications set the retrace time to approximately 17-20% of one horizontal period and the blanking time to approximately 20-25% of one horizontal period. These percentages are not appropriate, however, for high-resolution monitors which use a high horizontal scanning frequency, due to a characteristic limitation of the output transistor of the horizontal deflection circuit.
It is therefore important to determine and maintain the proper blanking time with respect to retrace time, which is dependent on the horizontal frequency employed. For example, should the retrace time be reduced for a higher horizontal frequency or should the blanking time be increased for a lower horizontal frequency, the horizontal deflecting current increases which increases power consumption and tends to overheat the output transistor. Also, shorter retrace times result in high voltage peaks in the output of the horizontal deflection circuit, which also is harmful to the output transistor. Meanwhile, the retrace time should be set with regard to the highest horizontal frequency used in a given display device, such that the deflection current, which is increased in a relatively low frequency mode to achieve full video deflection, may be too high for the output transistor.
One prior horizontal deflection circuit for providing a horizontal retrace time adjustment is contemplated by U.S. Pat. No. 4,705,993 to Shigeru Kashiwagi and entitled Horizontal Deflection Circuit Having A Variable Retrace Period utilizes a switching device which has a drawback of requiring user intervention by necessarily requiring the user to manipulate a switch for intermittently coupling a second capacitor to a first capacitor in a resonant circuit to vary the retrace period.